I don't believe the 230/250 engines are "interference" engines, where the valves and pistons can get together even if a valve stays open at the wrong time. Yes, you can take the valve cover off (not the "head") and soak the valve stems with penetrating oil, then use a pry bar to work the valves up and down. There's even a special tool made to compress the valve springs on those stud-mounted rocker arms, but I've always made my own tool by welding a flat washer onto a discarded rocker arm and using a long tapered punch as a lever.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
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